1. The Field of the Invention
The present invention relates to a biodegradable polymeric material essentially consisting of, or based on, thermoplastic starch, and to a polymer mixture comprising thermoplastic starch, a process for preparing a biodegradable material, a process for preparing a polymer mixture, and also to uses of the biodegradable material and of the polymer mixtures comprising thermoplastic starch.
2. Relevant Technology
Biopolymers based on renewable raw materials which are suitable for preparing biodegradable materials (BDM) are largely based on starch and comprise in particular thermoplastic starch, and also polymer mixtures made from thermoplastic starch and from other degradable polymeric components, such as polylactic acid, polyvinyl alcohol, polycaprolactone, tailored copolyesters made from aliphatic diols and from aliphatic or aromatic dicarboxylic acids, and also degradable polyester amides, which, with thermoplastic starch in an anhydrous melt via ester reactions and/or as polymer combinations form new degradable polymeric materials with a high proportion of renewable raw materials. There may be addition of other naturally occurring materials as additives and plasticizers such as glycerol and its derivatives, and hexahydric sugar alcohols such as sorbitol and derivatives of these.
EP 397 819 has for the first time specified a process for preparing TPS and also defined the new starch material known as thermoplastic starch (TPS) and specified the important differences, in particular in plastics processing technology, from the destructured starch which has been known for a relatively long time.
The thermoplastic starch is prepared with the aid of a swelling agent or plasticizer, not only without adding water but more particularly using dry or dried starch and/or starch which has been dried by devolatilization during the extrusion process while in the melt. Starches in the form of native starches commercially available comprise 14% of water, and potato starch as much as 18% of natural moisture at equilibrium.
If a starch with more than 5% of moisture is plasticized or pasted with exposure to temperature and pressure, this always gives a destructured starch. The preparation of the destructured starch is an endothermic procedure.
In contrast, the preparation of the thermoplastic starch is an exothermic procedure. In this case the essentially anhydrous ( less than 5%) native starch is homogenized in an extrusion process with an additive or plasticizer (e.g. glycerol, glycerol acetate, sorbitol) which lowers the melting point of the starch, and is melted within a temperature range of from 120 to 220xc2x0 C. by introducing mechanical energy and heat. The thermoplastic starch is free from crystalline fractions, or at least the crystalline fractions are less than 5% in the TPS, where the crystalline fractions remain unchanged and very small. The parameters of the process bring about a permanent rearrangement of the molecular structure to give thermoplastic starch, which now comprises practically no crystalline fractions and, contrasting with destructured starch, does not now recrystallize.
In destructured starch, the crystalline fractions immediately after preparation are likewise small, but these increase again when destructured starch is stored. This feature is also apparent in the glass transition temperature, which for thermoplastic starch remains at xe2x88x9240xc2x0 C., whereas in destructured starch, in contrast, it rises again to above 0xc2x0 C.. For these reasons, destructured starch and materials or blends based on destructured starch gradually become relatively brittle on storage, and, depending on temperature and time elapsed, the stresses contained within the polymer cause creep and distortion of the material (memory effect).
A differentiation of destructured starch and thermoplastic starch is:
When polymer mixtures based on thermoplastic starch are prepared, compatibilizers are used to homogenize the hydrophilic and polar starch polymer phase and the hydrophobic and nonpolar other polymer phase, which are either added or preferably are produced in situ (e.g. by transesterification) at specified temperature and shear conditions, to give processable granules. The technology of preparing these thermoplastic blends involves coupling together the phase boundaries between the low-compatibility polymers in such a way as to achieve the distribution structure of the disperse phase during processing via the ideal range of processing conditions (temperature and shear conditions).
The twin-screw extruders which, for example, are used for the compounding are preferably corotating twin-screw extruders with tightly intermeshing screw profile and kneading zones which can be individually temperature-controlled. The twin-screw extruders used for the TPS compounding or preparation of TPS/polymer blends preferably have eight compartments or zones which where appropriate may be extended to ten zones and have, for example, the following construction:
Extruder design: Corotating twin-screw extruder, for example:
Outside the extrusion plant: cooling and conditioning of the extrudates, where appropriate absorption of from 0.3 to 4% of water as plasticizer in a water bath, extrudate granulation, and bagging.
The extrusion conditions given above for preparing thermoplastic starch or mixtures based on thermoplastic starch are substantially directed toward the example of a TPS/PCL (polycaprolactone) polymer mixture. The processing or extrusion conditions change, of course, for polymer mixtures of other types. The example given above is intended merely to show how the prior art prepares polymer mixtures based on thermoplastic starch.
In connection with the original German Patent Application DE 19624641.5, in which the present invention was presented for purposes of a priority application, the relevant search by the German Patent Office mentioned the following publications: Kunststoffe 82 (11), pp. 1086-1089 (1992), W095/33874, W094/28029, W094/03543, EP 580 032, EP 404 727, U.S. Pat. No. 5,453,144, U.S. Pat. No. 5,321,064 and U.S. Pat. No. 5,286,770. All of the publications mentioned, insofar as they refer to starch, relate to native starch or destructured starch, i.e., they have no connection with thermoplastic starch of the type defined at the outset.
In the case of all of the polymer mixtures or polymers described in the prior art and comprising thermoplastic starch or based on thermoplastic starch, the assumption is made that the thermoplastic starch is initially created by conversion from native starch with a very substantially low-molecular-weight plasticizer or swelling agent. In the example given above, the TPS is prepared in Zones 1 to 4. Only subsequently, where appropriate, are other components mixed either purely physically or even to some extent chemically with the thermoplastic starch prepared in this way. In the example given above, an esterification or a transesterification reaction takes place during mixing of the PCL and the TPS, and the homogenization therefore also includes a chemical reaction. The additives and plasticizers which have been proposed and used hitherto, and which lower the melting point of the starch and have adequate solubility parameters, are, as mentioned, low-molecular-weight additives such as, inter alia, DMSO, butanediol, glycerol, ethylene glycol, propylene glycol, a diglyceride, diglycol ether, formamide, DMF, dimethylurea, dimethylacetamide, N-methylacetamide, polyalkene oxide, glycerol mono- or diacetate, sorbitol, sorbitol esters and citric acid.
Use has also been made on occasions of PVOH, EVOH and derivatives of these, and also of urea and urea derivatives.
In the original Patent EP 397 819, the solubility parameter of the plasticizer must be in the required range in order that the function is fulfilled. This is the important factor in the preparation of the thermoplastic starch, that the water removed is substituted by a plasticizer, so that the decomposition temperature of the starch is lowered on conversion to thermoplastic starch or thermoplastically processed starch to a sufficient extent so that the mixing in the melt takes place below the relevant decomposition temperature of the starch.
Entirely unexpectedly, it has now been found that polymers, such as polyester amides, aliphatic polyesters and copolyesters, and also a large number of other polymers specified below, can take on this function. This now brings with it the important advantage that when, in particular, polymer mixtures are prepared which are based on thermoplastic starch or TPS, the TPS does not firstly have to be prepared by conversion from native starch using a low-molecular-weight plasticizer before the other polymer is metered into it. In contrast, the polymer mixture can be prepared directly in what amounts to a single operation, by mixing native starch or starch derivatives with the additional preferably biodegradable hydrophobic polymer under dry conditions in the melt, where the starch present therein is thermoplastically processable. This therefore dispenses with the requirement for initial admixing of a low-molecular-weight plasticizer, such as glycerol, which the prior art of necessity proposes.
According to the invention therefore a polymeric material is proposed, essentially consisting of, or based on, thermoplastic starch. The thermoplastic starch therefore comprises, as plasticizer or swelling agent very substantially responsible for converting native starch or derivatives thereof into thermoplastic starch, at least one hydrophobic, preferably biodegradable, polymer.